ハザマ研究年報（2006.12） 1

１. Introduction

A technique using atmospheric pressure as a temporary

surcharge was principally proposed by Kjellman 50 years ago (Kjellman, 1952), and was worked out by the Royal Swedish Geotechnical Institute as a method for fine grain soil improvement. The method is based on the idea of applying vacuum suction to an isolated soil mass to reduce the atmospheric pressure in it, thus by the way of reducing the pore water pressure in the soil the effective stress is increased without changing the total stress.

Figure 1 Swedish vacuum method (Kjellman, 1952) However, for several following decades the method had

not widely been applied due to various difficulties, mainly in maintenance of effective vacuum pressure during treatment. With improvement in the methodology and

producing high quality vertical drains and airtight sheets, vacuum consolidation method became popular in many countries. Furthermore, together with clarifying the actual mechanism of vacuum preloading, improving construction techniques for drain installation and vacuum pumping operation, monitoring process, as well as developing analytical methods for designing allowed the method be applied effectively and economically and capable to conduct in various site conditions whether on-land or underwater. Even that, to make the method more beneficial, researches are kept continuing in order to increase its efficiency and to extend to various application fields. Various international symposiums on vacuum consolidation have been conducted for the purpose of better understanding of the technique and technology transfer.

Beside Sweden as pioneer in this field of technology, great contributions in development and advancing vacuum consolidation method to various fields of applications come from China, the U.S., Japan and other European and Asian countries. Most recently, a Japanese group has successfully developed a new technique for heightening the vacuum pressure in the soil. Learning experience from

Vacuum Consolidation Method – Worldwide Practice

and the Latest Improvement in Japan

Loan T.K.DAM＊・Isamu SANDANBATA＊＊・Makoto KIMURA＊＊

ABSTRACT: Vacuum consolidation method is a technique of applying vacuum suction to an isolated soilmass to reduce the atmospheric pressure in it, thus by the way of reducing the pore water pressure in the soilthe effective stress is increased without changing the total stress. By improving construction techniques, aswell as developing analytical methods for designing, the technique has become an effective and economicalmethod for soft ground improvement and capable to conduct in various site conditions. Researches are alsoextended to other application fields such as acceleration dewatering and consolidation of fluid-like materials.Conducting of numerous vacuum consolidation projects has been tried in many countries in the world, through that practical experiences are accumulated, which would have significant importance in improving its efficacyand advancing the technique. For the purpose of gaining experience, numbers of published documents andresearch papers on vacuum consolidation practice from leading countries in this field of technology have beenreviewed. This paper summarizes and compares various vacuum consolidation systems with emphasizing on system specification and configuration, design and construction practice, and introduces the newestimprovement of this technology in Japan.

Keywords： soil improvement, vacuum consolidation, surcharge, vertical drain, horizontal drain

* Research Assistant, Hazama Corporation ** Hazama Corporation

報 告

ハザマ研究年報（2006.12） 2

those countries would have significant importance in advancing the technique and improving its efficacy in various application fields. This paper reviews vacuum consolidation practice from various countries with some comparison in the system configuration, design and construction, and introduces the newest improvement in Japan.

2. Various Vacuum Consolidation Systems and

Construction

Various vacuum consolidation systems are developed

for number of applications. As primary application of vacuum consolidation method is consolidation and stabilization of soft clayey ground, most of the systems developed for these purposes are basically similar to the one originally developed by Norwegian Geotechnical Institute (NGI), though every system may have some specific features. When the vacuum consolidation method is extended to other applications with different working conditions and different treated materials, the systems require specific features and/or special designed construction machines. Moreover, vacuum consolidation applied in on-land and under-water conditions are principally different in equivalent load as the vacuum is applied on ground surface or under water table, relatively. 2.1 On-land vacuum consolidation system for soft

ground improvement

Principally, a vacuum consolidation system consists of a system of drains vertically installed from ground surface into the treated soil mass to prescribed depth, a surface drainage system including a granular medium (sand mat) and horizontal drains, and collector pipes leading to a vacuum pump system for transmission of vacuum to the soil as well as discharging water and air out of the treated soil mass. The vacuum treated soil mass is isolated from surface by an airtight membrane and if required laterally protected from leakage by cut-off-walls. Table 1 presents three typical vacuum consolidation systems utilizing different types of vertical drains. (1) Vertical drains

In any on-land system, numbers of drains are installed vertically from ground surface in triangular or square grid pattern at selected spacing into the treated ground to

prescribed depth. The vertical drains can be different in shape, material, structure and drainage properties. Typical types of drains are prefabricated board drain (familiar in ground improvement under name of “PVD”), cylindrical pipe drains (named as Menard vacuum transmission pipe, VTP) and sand drains. PVD is the most popular type of drains used in vacuum consolidation nowadays. Sand drains have been used since early development of vacuum consolidation in China and some other countries, however due to various disadvantages this type of drain has been almost completely replaced by the PVDs.

As the drain is one of the most important things that make the method to be technically and cost effective, the development of drain material is continued. Japan introduced newly developed highly permeable drain material for the PVD. Moreover, considering environmental impacts, some kinds of degradable drains such as bamboo drains (Singapore) and biodegradable plastic drains (Japan) are introduced but still not popularly employed in vacuum consolidation.

These systems are basically for on-land sites, although there are a few cases applying to shallow under-water condition (Sasaki, 2002). (2) Surface drainage

Normally it consists of a granular layer (commonly a sand mat) and a system of perforated collector pipes with/without interconnected horizontal drains embedded in. Horizontal drains connect the vertical drain tops to the main vacuum pipe. The type and layout of surface drainage system can be modified in various systems. The sand mat layer is commonly of 0.3-0.8m in thickness (depending on sand permeability, selected spacing of vertical drains and surface trafficability), though occasionally it maybe thicker to serve as working platform. Occasionally, the sand mat might be omitted and replaced by drainage geotextile layers (Japan). Horizontal drains can be different in types (corrugated flexible PVC pipes or PVD board drains), as well as in arrangement. See Table 1 for different surface drainage arrangements in Chinese and Menard systems. (3) Sealing techniques for vacuum isolation in treated soil mass

The airtightness of the system strongly influences the attained vacuum pressure and the efficiency of the system.

ハザマ研究年報（2006.12） 3

Tab

le 1

O

n-la

nd v

acuu

m c

onso

lidat

ion

syst

ems f

or so

ft gr

ound

impr

ovem

ent

Sys

tem

s usi

ng

PV

D S

yste

m u

sing

cyl

indrical

pip

es

Sys

tem

with V

ert

ical

San

d D

rain

s

(A

fter

TP

EI re

port

, Chi

na 1

995)

(ww

w.M

enar

d-Sol

trai

tem

ent.co

m)

Mod

ified

aft

er V

an M

ighe

m 1

999

(ww

w.ter

ra-e

t-aq

ua.c

om)

Chin

ese s

yste

m a

nd

oth

er

sim

ilar

syst

em

s in

the U

S.,

Menar

d M

VC

(Fra

nce)

deve

lope

d an

d em

plo

yed

byD

eve

lope

d by

SIL

T N

V (

Belg

ium

) si

nce 1

990s'

Euro

pe a

nd

Asi

a countr

ies

(inclu

ding

Jap

an s

yste

m)

Menar

d Soltra

item

ent

in F

rance

and

fore

ign c

ountr

ies

Use

d in

Belg

ium

Princ

ipal

ly w

ork

as

origi

nal

desi

gned

syst

em

Princip

ally

work

as

origi

nal

desi

gned

syst

em

Vert

ical

san

d dr

ains

of

larg

e d

iam

ete

r;M

ost

com

mon t

ype o

f dr

ains

PV

D, hig

h-sp

eed

inst

alla

tion

Menar

d cyl

indr

ical

dra

ins

of

hig

h d

ischar

ge c

apac

ity

Use

nat

ura

l bo

ttom

san

d lay

er

as h

orizo

nta

l dra

inag

eThe m

em

bran

e is

cove

red

with w

ater

to s

ust

ain h

igh

App

lyin

g pr

imar

y fill

beneat

h m

em

bran

e t

o incre

ase

and

for

appl

ying

vacuum

pre

ssur

e;

vacuum

pre

ssure

and

preve

nt

the m

em

bra

ne a

ging…

seal

ing

and

stab

ility

of sy

stem

.V

ert

ical

dra

inag

e d

ow

nw

ard;

Indi

vidu

al t

reat

ment

area:

sta

nda

rd 6

,000 -

10,0

00 m

2In

divi

dual

tre

atm

ent

area:

sta

nda

rd 5

,000

-7,0

00 m

2

Conve

ntional

const

ruction m

achin

es

Vert

ical

dra

inag

eP

VD

(car

d bo

ard

drai

n), 1

00 m

m x

3-4 m

m, sp

acin

g 1.0

-1.2

M

enar

d c

ylin

drical

pip

es

VTP

, φ5

0m

m; sp

acin

g 1.4

-1.

5m

Vert

ical

san

d dr

ains, φ3

00 m

m, sp

acin

g 2.7

5 m

, cap

ped

Horizo

nta

l dr

ainag

eSan

d m

at +

man

ifold

fro

m m

ain f

ilter

colle

cto

r pi

pes

with

San

d m

at +

perf

ora

ted

PV

C p

ipes

in t

wo p

erp

endi

cula

rB

ott

om

nat

ura

l sa

nd

laye

r.fish

-bo

ne b

ranches

PV

C p

ipes

(Chin

a) o

r P

VD

(Jap

an)

dire

ctions

connecte

d to

periph

era

l colle

cto

r pi

pe;

Surf

ace m

em

bra

ne

PV

C a

irtigh

t sh

eet

(one o

r se

vera

l la

yers

);P

VC

wove

n g

eote

xtile

mem

bran

e;

Mem

bran

e is

not

requ

ired

as s

and

drai

ns

are c

appe

dP

eriphera

l tr

ench

Cla

y m

ix s

lurr

y (C

hin

a) o

r in

-si

tu im

perm

eab

le s

oil

(Jap

an)

Bento

nite a

quak

eep

slurr

y an

d ba

ckf

ill w

ater

on t

op;

on t

op b

y cla

y an

d si

lt;

Addi

tion

al s

eal

ing

Cla

y re

vetm

ent

above

the t

rench for

reta

inin

g so

il fill

Prim

ary

fill

(1.5

m)

on t

op

of

sand

beneat

h m

em

bran

e;

or

ext

racte

d w

ater

disc

har

ged

above

the m

em

bran

e;

Lat

era

l confinin

gC

lay

mix

ed

cut-

off-w

all;

Bento

nite s

lurr

y w

all or

sheet

pilin

g;φ

48Jet

pum

p + 3

HA

-9 c

entr

ifuga

l w

ater

pum

p of

7.5

kW;

Menar

d va

cuum

pum

p s

tation M

S25 (25 k

W)

2 r

ow

s of

vacuum

pum

ps a

rran

ged

along

the s

ides

Vac

uum

pum

p1000-1500 m

2/pu

mp,

vac

uum

pre

ssure

>80 k

Pa

(Chin

a)5,

000-7,0

00 m

2/pu

mp

stat

ion, desi

gned

vacuum

75 k

Pa

of

impr

ove

d ar

ea

(rai

lway

founda

tion)

specific

atio

nA

ir-w

ater

sepa

ration h

igh v

acuum

pum

p s

yste

m (Jap

an)

2000-2500 m

2/pu

mp,

effective

vac

uum

pre

ssure

>80 k

Pa

Inst

rum

enta

tions

Auto

mat

ic c

ontr

olli

ng,

record

ing

and

meas

urin

g sy

stem

Auto

mat

ic c

ontr

olli

ng,

record

ing

and

meas

uring

syst

em

Not

specifie

dV

ario

us

proje

cts

of

port

s, a

irpo

rts,

hig

hw

ays,

bridge

s,V

ario

us

proje

cts

of

road

s & h

ighw

ays,

bridge

s, b

uild

ings

,Seld

om

use

d now

aday

s, h

ow

eve

r be

neficia

l in

spe

cia

lbu

ildin

gs, pe

trole

um

, chem

ical

, se

wag

e &

pow

er

stat

ions.

.. pe

trole

um

and

pow

er

stat

ions,

sew

age fac

ilities…

condi

tions

for

soft

gro

und

impr

ove

ment.

The E

ast

Pie

r Xia

ng

Port

(Tia

njin

, C

hin

a 1987)

Road

837 (

Fra

nce, 1994)

Rai

lway

founda

tion S

poorw

egz

ate (B

elg

ium

, 1999):

San

riku

Moto

rway

pro

ject

(Tohoku

, Jap

an 2

003)

Kim

hae S

ew

age t

reatm

ent

facili

ty (

S.K

ore

a, 1

995)

impr

ove

ment

of

soft

dis

posa

l se

dim

ent

(Slu

dge)

Spe

cia

l fe

ature

s

System configuration

Applic

atio

ns

Typ

ical

pro

ject

Sys

tem

typ

e

Imag

e

Typ

ical

sys

tem

Dra

ins

Wat

er

Rev

etm

ent

Tren

ch

Jet p

ump

Filte

rPi

ping

Sand

Mem

bran

e

P

erim

ete

rcolle

cto

r pip

eTre

nch

Horizo

nta

l pi

pedra

ins

Dra

inconnecto

rs

Very

soft

fine-gr

ain

soilS

and p

latf

orm

Nat

ura

l San

dV

acuum

pum

p

Ext

ractio

n w

ell

Cla

y + P

eat

Cla

y + P

eat

ハザマ研究年報（2006.12） 4

Table 2 Various types of drain utilized for vacuum consolidation

The common practice is using PVC airtight membranes (2-3 layers) to cover the entire treatment area and they are welded at the site. China developed single air tight membrane layer more than 100,000m2. A geotextile layer may be laid on the ground surface before covering by membrane to avoid damage. To complete the sealing, the membrane edges are keyed in peripheral trench excavated to the depth at least 0.5m below the ground water level and filled with impervious slurry, which can be clay mix slurry (China), Bentonite Polyacrolyte slurry with water on top (French Menard), or in-situ excavated clayey soil (Japan).

Beside that, different systems may utilize different technique for additional sealing. Among that is a practice of construction of compacted clay revetment above the trench to retain the water/fill placed on the top of the membrane. This placement of water is useful not only in increased sealing of the system, but also in preventing aging of the membrane and minimizing damage from traffic and wildlife, as well as affecting as a surcharge. Another technique is from Menard system, in which a 1.5m thick primary fill is constructed directly on the top of the sand mat beneath the membrane for increasing the stability and sealing of the system. The fill will maintain a non-submerged action beneath the membrane even when it has been settled below the original ground water level, therefore the vacuum intensity will not decrease during treatment (Cognon et al., 1996).

In case of pervious soil layer exists near the ground surface, the common technique for lateral confining is construction of cut-off-walls. According to China experience, a clay mix slurry wall (0.7m thick, permeability <1x10-5 cm/s) constructed by the deep mixing method throughout pervious layer and at least 1.5m inside the clay layer would be effective. Other types

of material such as Bentonite slurry (Menard) or specialGeolock (Holland BV) are also applicable. (4) Vacuum pump system

Generally, a high efficiency vacuum pump system equipped with discharge pump is used to provide suction to the soil and to discharge the air-water out through the system of pipes and drains.

In China, the common vacuum pump has been replaced by the φ 48mm Jet Pump (7.5 kW) with 3HA-9 centrifugal water pump, which can generate a vacuum pressure greater than 90 kPa. For Menard system, a vacuum station consists of a specifically designed high-efficiency vacuum pump acting solely on the gas phase in conjunction with a conventional vacuum pump allowing liquid and gas suction. In Japan, Maruyama Industry Ltd. and Hazama Co. group successfully employed a specially designed vacuum pump system, which can separate water and air collected in the main and sub-separator tanks by means of build-in discharge water pumps, thus sustaining high under-sheet vacuum pressure during treatment. (5) Instrumentations

In any vacuum consolidation system, various instrumentations are necessary to control the system operation as well as to monitor the performance of the treatment including vacuum pressure, optimal pore water pressure, discharged water volume, settlement and lateral displacement, which are useful in judgment of the time to stop the vacuum pump, and controlling stability of embankment construction.

2.2 Vacuum consolidation systems for dewatering

fluid-like materials

General Spacing 1.4～1.5m depending on ch

more than 10 m

China: 1.2m (drain diameter 70 mm)

Materials ・Grooved polypropylene core and non-woven textile filter ・Special geotextile protection, filter Fine Sand

・High conductivity

・High cost performanceChina:1.2～1.3 m depending on ch

Japan:0.7～1.2 m depending on cv

Special Futures

・High conductivity k=7.0×10-2 cm/s・The most major type・High integrity preservation for dynamicforce

・The least disturbance

Round, various diameter (70 - 300 mm)Rectangular 100 mm×3～4 mm Round, diameter 50 mm

China and some other countriesMajor CountriesChina, U.S.,

European and Asian countriesFrance and some other countries

(Designed by Menard)

(PVD)

Applicable Depth more than 40 m more than 50 m

Drain Type

・Plastic cylindrical core with grooves

・Easy and high-speed installation

Prefabricated Board Drain

Belgium: 2.7m (drain diameter 300 mm)

Cross-section

(VTP:Vacuum Transmission Pipe) Sand Drain

・Common used before, rarely nowadays・High conductivity・Difficult to obtain and maintain quality

Cylindrical Pipe Drain

ハザマ研究年報（2006.12） 5

Table 3 Types of vacuum systems

In order to apply for accelerated dewatering and consolidation of new hydraulic fills, sewage and slug, lagoons and mine ponds mud etc. vacuum consolidation system with special features to deal with fluid-type materials and special construction machines capable to work whether on-land or underwater conditions are necessary. Typical vacuum consolidation systems applied for acceleration dewatering and consolidation of fluid-like materials are illustrated in Table 4. (1) Systems with vertical drains

Originally, the US developed vacuum consolidation system (since 1970s) for treating hydraulic fills consists of multi-stage dike constructed on the surface of natural soil for containing dredged disposal (on-land confined disposal facility; CDF). During dumping of hydraulic fill, horizontal drainage layers (drainage sheets or sand) are placed. Several vertical drainage wells are constructed thereafter and connected to vacuum pump through a system of slotted collector pipes which can be arranged on the surface or at the bottom of the CDF depending on whether drainage is upward or downward, respectively. Later, the USACE (US) modified the system by using PVDs instead of vertical drainage wells for applying in underwater condition for dewatering contaminated hydraulic dredged materials at Newark Bay Sub-aqueous CDF project. As the vertical drains are capped at tops and drainage was allowed downward using submerged vacuum pump, membrane was not required for sealing (Thenavagayam, 1996).

Beside that, underwater vacuum systems utilizing vertical drains for consolidation of soft seabed soils in offshore condition include the one investigated at the

Imperial College, London proposed for submarine consolidation at Chek Lap Kok, Hongkong (Harvey, 1997) and the one recently developed in Japan (Shinsha et al., 2002). Those systems are characterized by a system of vertical drains installed throughout underwater soft seabed soils with the tops of the drains being capped and buried in the soil and connected to a pipe system leading to vacuum pump, so that membrane for sealing is not required. (2) Systems with lateral drains

As the treated materials are very soft to fluid-like, specially designed construction device for working in water condition and to inserting the drains horizontally in the soil is necessary. Typical systems of this type are those developed in Japan and Belgium.

Penta-Ocean Co. (Japan) developed a system using special designed floating boat with a winch and a guide leader to install simultaneously several (generally 4) card-board drains laterally and parallel in layers at the spacing of 0.7-1.5m within the new hydraulic fill. With the boat cruising back and forward, the installation is completed. The installation leaves a 1.5m thick surface layer of treated material above the upper drains to serve as natural sealing for the system. Each lateral drain, which may be 300m in length, has a hose to connect to the vacuum pump through a header. The technique is limited by the thickness of treatment of 5-6m as only few drain layers can be employed. Its effectiveness for dewatering and consolidation has been verified in several cases including an actual improvement project of slurry-like soda ash at a disposal site of 40,000m2 (Shinsha et al, 1996).

System Type Chinese Type Menard MS5 type Japanese Type

Image

・Generated vacuum pressure: up to 90 kPaAir-water separation high vacuum systemVacuum Pump

Specification Power 7.5 kW Power 25 kW

・Generated vacuum pressure: >90 kPaφ48Jet pump + 3HA-9 centrifugal wate

・Generated vacuum pressure: 80 kPaMenard MS25 type

Able to separate water and air, whichresults in high vacuum performance intreated area. One vacuum pump responsiblefor 2000-3000 m2 treated area.

Special designed vacuum pump actingsolely on the gas phase in conjunction withconventional vacuum pump allowingliquid and gas suction. A vacuum pumpstation is responsible for 5000-7000 m2.

One vacuum pump responsible for atreatment area of 1000-1500 m2

Special Futures

ハザマ研究年報（2006.12） 6

Tab

le 4

V

acuu

m C

onso

lidat

ion

Syst

ems f

or A

ccel

erat

ed D

ewat

erin

g of

Flu

id-li

ke m

ater

ials

Sys

tem

with P

VD

, dow

nw

ard d

rain

age

Sys

tem

with P

VD

, upw

ard d

rain

age

Sys

tem

with h

orizo

nta

l P

VD

dra

ins

Sys

tem

with h

orizo

nta

l pip

e d

rain

s

(Aft

er S

andi

ford

et

al.,

1996

)(A

fter

Har

vey,

199

7)(A

fter

Shi

nsha

et a

l, 19

96)

(Aft

er V

an M

iegh

em e

t al

., 19

99)

Deve

lope

d by

USA

CE (U

.S.,

1996) ac

cord

ing

to1) Im

perial

Colle

ge, Londo

n (1997) pr

opo

sed

Deve

lope

d by

Penta

-O

cean

(Jap

an)

Deve

lope

d by

SIL

T N

V (B

elg

ium

)

origi

nal

sys

tem

with v

ert

ical

and

horizo

nta

l dr

ains

for

appl

icat

ion a

t C

heck

Lap

Kok

(Hong

Kong)

since 1

987-1988

deve

lope

d in

the U

S s

ince 1

970s.

2) P

enta

-O

cean

Co. Jap

an (re

cently)

App

licab

le for

unde

rwat

er

or

on-la

nd

condi

tions

Unde

r w

ater

syst

em

, offsh

ore

condi

tion

App

licab

le in o

ffsh

ore

or

on-la

nd

condi

tions

Offsh

ore

condi

tion a

t w

ater

dept

h fro

m 1

-25m

Vert

ical

dra

inag

e d

ow

nw

ard;

Vert

ical

PV

D (A

lidra

ins)

cap

ped

at t

op

and

Horizo

nta

l dr

ains,

eas

ily a

nd

relia

bly

inst

alle

dH

orizo

nta

l dr

ainag

e

Require initia

l const

ruction o

f du

mpin

g fa

cili

ty a

nd

buried in t

he s

oft

seab

ed

cla

y, inst

alle

d do

wn

horizo

nta

lly a

nd

para

llel in

4 lay

ers

usi

ng

Use

spe

cia

l de

sign

ed

lay-

barg

e e

quip

ped

with

engi

neering

plac

em

ent

of bo

ttom

dra

inag

e lay

er;

to t

he d

ept

h o

f st

iff cla

y.float

ing

barg

e w

ith w

inch a

nd

guid

e lead

er.

plough

for

inse

rtin

g dr

ain t

ube

s horizo

nta

lly.

Wat

er

dept

h 1

0m

Tre

atm

ent

dept

h: 5-6m

Hig

h s

peed

and

eas

ily inst

alla

tion

Vac

uum

pre

ssure

50 k

Pa

(supp

ose

d)Sett

lem

ent: u

p to

50% o

f in

itia

l th

ickn

ess

;

Aft

er

treat

ment

w0 =

80-90% w

L

Not

requ

ire m

em

bran

e for

surf

ace s

eal

ing

Not

requ

ire m

em

bran

e for

surf

ace s

eal

ing

Not

requ

ire m

em

bran

e for

surf

ace s

eal

ing

Not

requ

ire m

em

bran

e for

surf

ace s

eal

ing

Vert

ical

drai

na g

eV

ert

ical

str

ip d

rain

s, c

appe

d at

top,

spa

cin

g 1.5

mV

ert

ical

Wic

k dr

ains

buried

in t

he s

eab

ed

cla

yN

ot

use

dN

ot

use

d

Engi

neering

sand

laye

r w

ith e

mbe

dded

drai

n p

ipes

Horizo

nta

l pi

pe s

yste

m c

onnecting

all ve

rtic

al

Horizo

nta

l bo

ard

drai

ns

spac

ing

0.7

-1.5

mTw

o lay

ers

of horizo

nta

l dr

ain t

ube

s, s

pacin

g 1.5

m

inst

alle

d at

the b

ott

om

of C

DF b

efo

re p

lacin

g dr

ains

to s

ubm

erg

ed

vacuum

pum

pin

stal

led

within

dre

dged

fill

afte

r fill

plac

em

ent.

connecte

d to

colle

cto

r pi

les

led

to v

acuum

pum

p

dredg

ed

mat

erial

.

Surf

ace

seal

ing

A lay

er

of hyd

raulic

fill

left

abo

ve t

he d

rain

s' t

ops

A s

oft

soil

laye

r ab

ove

the c

apped v

ert

ical

dra

ins

Top

soil

laye

r of 0.6

5m

thic

kTop

silt lay

er

0.5

-1.0

m t

hic

k

Add

itio

nal

seal

ing

Cap

ping

the v

ert

ical

dra

ins

Cap

ping

the v

ert

ical

dra

ins

Vac

uum

pum

pSubm

erg

ed

vacuum

pum

pSubm

erg

ed

vacuum

pum

p sy

stem

On-sh

ore

Vac

uum

pum

p an

d su

bmerg

ed

disc

har

ge p

um

p

Dew

ater

& c

onso

lidat

e d

um

ping

soils

to incre

ase

Conso

lidat

e a

nd s

tabi

lize o

ff-sh

ore

soft

mar

ine

Dew

atering

and

conso

lidat

ing

hydr

aulic

dre

dged

Dew

atering

and

conso

lidat

ion o

f dr

edg

ed

silt t

o

stora

ge c

apac

ity

of confinin

g di

sposa

l fa

cili

ties.

cla

ys in r

ecla

mat

ion p

roje

ct

fills

or

slurr

y-lik

e w

aste

s in

cre

ase t

he s

tora

ge c

apac

ity

of di

sposa

l fa

cili

ty

Feas

ibili

ty inve

stig

atio

n for

apply

ing

in p

roje

ct

atExp

erim

enta

l tr

ial fo

r dr

ain a

pplic

atio

n feas

ibili

tyExp

erim

enta

l tr

ial at

recla

imed h

ydra

ulic

lan

dfill,

Actu

al t

reat

ment

at A

nw

erp

Port

pro

ject

(Belg

ium

)

Sub-

aqueous

CD

F N

ew

ark

bay,

New

Jers

ey

(US)

at t

he s

ite o

f C

heck

Lap

Kok

(Hong

Kong)

Tre

atm

ent

of

slurr

y-lik

e s

oda

ash

dis

posa

l si

te

Typ

ical

proje

ct

App

licat

ions

Imag

e

Typ

ical

syst

em

Spe

cia

lfe

ature

s

Configurations

Horizo

nta

l d

rain

age

Sys

tem

typ

eSys

tem

s w

ith v

ert

ical

dra

ins

Sys

tem

with H

orizo

nta

l D

rain

s

Ree

l

Hor

izon

tal d

rain

sG

uide

lead

e r

Dra

inag

eh o

se Ro

Anc

hor

Styr

ofoa

m

Win

cH

ydra

ulic

fill

Stiff

cla

y

To p

umpi

ng s

yste

m

Seab

ed, S

oft

mar

ine

clay

Sea

wat

er

Wic

k dr

ain

VA

CU

UM

PU

MP

Upp

er

drai

n

Low

er

drai

nTot

al le

ngth

Dre

dge

dsi

lt

Vac

uum

pum

p

New

ark

Bay

Sand

s

Var

ved

silt

san

d cl

ays

Shal

es a

ndsa

ndst

ones

Soft

Bot

tom

sand

lay

erw

ithd

ii

Ver

tical

drai

nsD

redg

edm

ater

ials

ハザマ研究年報（2006.12） 7

SILT NV (Belgium) developed an underwater vacuum consolidation system using special designed lay-barge with plough construction device to insert drain tubes horizontally in very soft silt. This makes the technique easier to execute and less expensive than the use of vertical drains. The drain tubes have to be inserted at least 1.0 m underneath the top of the silt to assure minimum leakage of water above the silt into the drain tube. Collector tubes are laid on the shore connecting all the drain tubes. A specially designed vacuum pump maintains pressure efficiency of 80-90% in the drains. The system can be applied in the condition of 1-25m water depth. The technique was employed in a project at the Anwerp Port in Belgium in 1996 for extracting water from basin of dredged silt over an area of 120,000m2, which brought a gain of 20% in storage capacity of the basin. (Van Mieghem et al., 1999). Furthermore, researches on application of this technique for accelerating consolidation and stabilizing underwater slopes and embankments are carried on. 3. Vacuum consolidation design and construction

experience

Principally, design of vacuum consolidation can be

based on consolidation theories (Terzaghi and Barron theories). Finite Element analysis (FEM) is used to model the soil stress-strain behavior. Design parameters (individual block area, sealing measures, drain spacing, depth, effective vacuum pressure and combined surcharge, vacuum pumping period…) are essentially dependent on the site conditions, soil properties, the used vacuum and drainage system, the purpose and other requirements of the project. Therefore, experience is of great importance in selection of optimum design to satisfy both technical and cost effectiveness. (1) Individual vacuum treatment area and the treatment area by single vacuum pump

Depending on size the total treatment area is divided into blocks for sealing in individual vacuum treatment. In Chinese practice, the block size normally varies at 6,000- 10,000m2, although it is feasible from minimum 1,000m2 to maximum 30,000m2. For each block, one or more vacuum pumps are equipped providing that an area for individual vacuum pump is standardized at 1,000-1,500m2.

However, in case of presence of thick pervious layer near the surface causing lateral leakage of vacuum, the individual treatment area should be reduced (to 600m2 according to experience from project of Yaoqiang Airport, China, Tang & Shang, 2000). For Menard design, the standard area for a Menard vacuum pump station is 5000-7000m2, and several vacuum pump stations will be used according to the size of treatment area. In Japan, however, the individual treatment area is specified at 2,000-2500m2, which will be supported by a single vacuum pump. (2) Drain spacing:

For each treatment, the vertical drain spacing has to be determined according to the coefficient of consolidation of the treated soil and the time requirement. Generally, optimum spacing should be selected by balancing between the time and the cost. Different vacuum consolidation systems utilizing different types of drains (PVD or cylindrical pipes) may find different range of optimum spacing.

According to Chinese experience from numerous projects in the Port of Tianjin, for most of the treated soils with ch ≅ 1-2.5 x 10-3 cm2/sec and the time available for vacuum pumping was generally 5 months, spacing of PVD drains was generally selected at 1.2-1.3m. However, it might be shortened to 1.0m for soils of ch ≅ 0.6-0.7 x 10-3 cm2/sec, or even to 0.5-0.85m in cases of restricted time (Liu, 1995). In Japan, it is specified that the PVD spacing should not larger than 1.2m, but from 0.7-1.2m corresponding to soil cv value. Actual practice in Japan indicated that the selected PVD spacing is mainly at 0.8-1.0m. The highest spacing for PVDs was 1.5-1.8m in the Pier 300 Port of Los Angeles project (Thenavagayam et al., 1996).

For Menard practice, spacing for cylindrical drains is commonly selected at 1.4-1.5m for most soils with ch = 0.2-2 x 10-3 cm2/sec, though it may vary from 1.0m to 1.7-1.8m in some projects depending on soil properties and other factors. (3) Effective depth

Depending on soil profile and required improvement, the drain length is determined. For most of projects in China and Japan, the treatment depth was within 20m, with exception cases up to 25-30m. However, the

ハザマ研究年報（2006.12） 8

maximum treatment depth up to 43m has been evident in Menard project at Kimhae Sewage Treatment Plant (South Korea), which suggests that the effective depth is only limited by considering the maximum depth to which vertical drains can be installed at reasonable price. Anyway, vertical drain must not penetrate into the permeable layer, but shall be terminated at about 1m above the bottom of the soft soil layer (Thenavagayam, 1996) or >0.5m according to TPEI China (Liu, 1996), while Japan recommends at least 2m considering uneven bottom of treated soil layer with regard of construction condition control. (4) Effective vacuum consolidation pressure

The effective vacuum pressure inside the soil (or so called under-sheet vacuum pressure) is normally lower than the vacuum generated in the pump. For on-land systems, efficiency of vacuum loading of 70-80% is evident. Based on actual experience, designed vacuum pressure in Menard MVC system is 75 kPa, while a value of 80 kPa is used in China design though greater values of efficiency (80-95 %) are reported. In Japan, a minimum value of 60 kPa used to be recommended for design. However, together with using highly airtight membrane and highly permeable drain material, introduction of special designed air-water separation vacuum pump system allows attaining a stable effective under-sheet vacuum pressure as high as 90 kPa during treatment (Sandanbata et al., 2004). (5) Determination of soil parameters

Compression index Cc, and coefficient of consolidations cv, ch are the most important soil parameters used in design for vacuum consolidation. In China, they are normally obtained from conventional laboratory consolidation tests on undisturbed samples. According to China experience, it is also recommended to use the Cc value form e-p’ consolidation curve for settlement calculation instead of using mv or Es converted from this curve (Liu, 1996).

The coefficient of consolidation cv and ch can be obtained from conventional consolidation tests on undisturbed samples, but sometimes is derived from calculation of field permeability coefficients and laboratory compression coefficients. However, when the parameters with high accuracy are required, the field

measured settlement-time curve at the early stage of consolidation is used to simulate fitting. (6) Consolidation degree, vacuum pumping duration

Generally, based on calculated final settlement and consolidation analysis, the vacuum pumping duration is determined so as to achieve a prescribed degree of consolidation and/or allowable settlement rate. The use of Finite Element method (FEM) in consolidation analysis and design is not widely used in practical works because of difficulty in obtaining satisfactory soil parameters, while experience showed that using Terzaghi and Barron theories could practically satisfy the requirement of most ordinary projects in China (TPEI, Liu 1995) as well as in Japan. For convenience, in many vacuum consolidation projects the ratio of observed settlement to calculated final settlement is adopted as the degree of consolidation of soil at the time (Liu, 1995). However, acknowledging that the effect of vacuum loading is controlled largely by pore water pressure changes, it is necessary to analyze the pore water pressure variations and use it for assessment of the degree of consolidation.

In Menard design, the period for vacuum pumping is determined as the time required to reach a “target settlement”, which is determined as 100% of primary consolidation plus 10 year of secondary consolidation under the designed load with a guarantee of 10cm post-treatment settlement over the next 10 years. Estimation of “target settlement” is based on laboratory primary and secondary consolidation curves and the actual settlement curve obtained during treatment.

Experience showed that in most vacuum consolidation treatment (treatment depth up to 20m) without or with small surcharge, the duration of vacuum loading varied from 3 to 5 months. However, it should be noted that, when vacuum is combined with high embankment loading (many cases in Japan) the vacuum pumping duration is normally extended in order to satisfy stable embankment construction at allowable rate. (7) Combined surcharge load

Combination of vacuum loading with surcharge to increase the applying load is widely employed. Surcharge fills up to 5-6m were reported from number of projects worldwide. China considered the maximum surcharge load to combine with vacuum is a problem of economical

ハザマ研究年報（2006.12） 9

0

4

8

12

16

20

0 1000 2000 3000 4000 5000 6000

S 2 / c h , (m 2 . se c / c m 2 )

No

rma

liz

ed

Du

rati

on

(d

ay

/1

mtr

ea

tme

nt

de

pth

)

Menard

China

Japan

(c)

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Comb i ned l oa d a pp l i ed ( k Pa )

Se

ttle

me

nt

rati

o,

S/

H (

%)

Menard, Cc/(1+eo)=0.26-0.28

Menard, Cc/(1+eo)=0.30-0.35

Menard, Cc/(1+eo)=0.45-0.47

Menard, Cc/(1+eo)= ?

China, Cc/(1+eo)=?

Japan, Cc/(1+eo)=0.20-0.27

Japan, Cc/(1+eo)=0.35-0.40

Japan, Cc/(1+eo)=0.45-0.50

Japan, Cc/(1+eo)=?

(d)

0

50

100

150

200

250

300

0 1000 2000 3000 4000 5000 6000

S2/c h (m2 . sec/cm2)

Pu

mpin

g D

ura

tio

n t

(day)

Menard

China

Japan

1215

121020

23 18

5.5

5.9

9.7

34 30

(b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 1 2 3 4 5 6

Coef f i c i en t o f cons o l i da t i on

chx 10- 3

( cm2/s ec )

Sq

ua

re o

f D

rain

sp

ac

ing

(S

2)

Menard projectsVTPChina projectsPVDJapan projects,PVD

(a)

Figures 2 Comparison of vacuum treatment design from various projects of Menard, China and Japan (Note: cv are used instead of ch in plotting Japan data set as they usually used in Japan designs)

comparison to be dealt with detailed situations. Maximum fill height up to 15.5m equivalent to 150kN/m2 surcharge was combined with vacuum loading in Menard project at Kimhae (South Korea).

On the other hand, the vacuum induced inward lateral compression is utilized to solve problems of excessive lateral displacement and instability of embankment foundation or slopes. In Menard project of construction Highway A837 Saintes-Rochefort, building up a 9m high embankment on very loose soil was made possible within less than 3 month. Experience in Japan proved that under support of high vacuum pressure, continuous rapid construction of a 12.5m high embankment could be completed successfully within 3 and a half months (Sandanbata, et al 2004).

Comparison of actual vacuum consolidation designs for typical systems utilizing PVD (China and Japan projects) and cylindrical VTP drains (Menard’s projects) are illustrated in Figures 2 (a-d). Typical designs of vacuum consolidation in number of projects are summarized in Table 5. The data were sited or interpreted from tables, graphs or texts from numbers of published documents and papers.

Fig.2 (a) compares the actual selected drain spacing S

(plotted in term of S2) and the range of average ch values of the treated soils. In general, distribution of data points from each data set (China, Japan, Menard) can be bounded by two lines, where the upper line represents the range of normal design whereas the lower one represents the cases of restricted construction time. It is clear that the selected drain spacing is not only dependent of soil properties, but also of the drain type and design criteria such as available time (for cost effectiveness). The actual pumping duration (generally considered as the time to reach about 90% consolidation degree) is plotted against the value S2/ch in Fig.2 (b) for various cases with each data point being labeled with its treatment depth. However, no clear trend was observed suggesting that in vacuum consolidation horizontal drainage might not be the unique process.

When pumping duration is normalized to the treatment depth as plotted in Fig.2(c), there are clearer trends approximately represented by a line for each data set. It suggests that drainage in vertical direction may also control the rate of vacuum consolidation. Different lines of different data point sets might indicate relative differences in design criteria or effectiveness of the systems. Also in cases of high embankment, the vacuum

ハザマ研究年報（2006.12） 10

Table 5 Typical vacuum consolidation designs from various projects

pumping duration is also affected by the embankment construction control to ensure its stability. It should be noticed that, distributions of Japan data points in Figs.2 (a, b, c) are used only for demonstration of the trends, but not for comparative purpose due to the fact that values of cv were used instead of ch.

Fig.2 (d) presents the performance of the vacuum treatment through plotting settlement ratio (total surface settlement S normalized to treatment depth H) against the combined applied load (approximately estimated as a total from vacuum pressure plus embankment surcharge height). Distribution of data points marked in accordance to average Cc/1+eo indicates that the settlement ratio ranged between 5-15% largely depending on both combine load and the soil compressibility.

4. VACUUM CONSOLIDATION PRACTICE AND

TYPICAL PROJECTS

4.1 Practices in China

In China the vacuum consolidation method has been investigated since 1960s and intensively studied including experimental researches and field trials since early 1980s. During 1982-1996, the method has been applied in total of 31 projects (with total area of 1.6 millions square meters) in the Port of Tianjin only, mainly for improving foundation soils for docks, storage yards, warehouses, sewage treatment facilities, oil tank farms, office buildings and living quarters… and about 20 projects in other areas of China (Liu, 1996). Since 1996 to present, the use of vacuum consolidation is extended to improvement of foundation of expressways, airport

Treated ResultedArea wo Cc/1+eo Ave. ch Sand Drain Treated depth Vacuum Fill Surcharge Pumping Settlement

(m2) (%) (ave.) x10-3cm2/s blanket(m) Spacing(m) Ave, (m) (kPa) (m) (m) duration (days) Ratio (%)Ambes1(France) 390 140-860 0.45 0.40 0.8 1.40 5.5 80 1.3 0.0 50 12.4Ambes2 (France) 21106 140-861 0.45 0.40 1.40 5.9 80 1.5 90 12.7Lomme (France) 8130 1.50 7.2 80 1.0 50 1.1Ambes3 (France) 17550 45-57 0.28 0.51 1.50 9.7 80 2.8 3.0 170 7.0Lamentin1(France) 17692 112-347 0.47 2.10 1.85 10 80 1.5 90 14.5Hamburg (Germany) 238000 180-250 0.20 0.5-1.0 12 80 5.5 60-150 29.2Ipoh Gopeng (Malaysia) 2600 70 1.30 1.50 12 80 7.0 90 5.8Bangbo (Thailand) 30000 105-130 0.30 0.48 1.20 18 80 3.5 150 12.8Kuching (Malaysia) 12000 60 0.26 0.90 1.20 15 80 3.0 2.0 65 4.0A837(1) (France) 44500 52-75 0.27 2.00 1.40 20 80 2.5 3.0 85 7.0A837(2) (France) 10000 60 0.27 2.00 1.2-1.45 23 80 3.3 140 7.1Wismar (Germany) 15000 80-250 1.20 19 80 1.5-2.5 0-8 90 9.3Jangyoo (S.Korea) 70000 80-150 0.27 0.80 0.8-1.5 30 80 3.0 0-7 270 14.5Kimhae (S.Korea) 83580 80-150 0.23 0.80 1.0 0.9-1.4 34 80 1.5 3-6.5 270 12.7

Area wo eo Ave. ch Sand Drain Treated depth Vacuum Fill Surcharge Pumping Settlement

(m2) (%) x10-3cm2/s blanket(m) Spacing(m) Ave, (m) (kPa) (m) (m) duration (days) Ratio (%)Guandong Express 14,461 26-90 0.7-2.4 2.60 1.5 30 80 0 4 165 5.7Xiang Port 480,000 44-64 1.2-1.7 0.8-1.5 0.4 1.3 18 80 0.7 135 8.9Xiang Port 80 0.7 1 170 12.8Yaoqiang Airport 145,000 22-48 0.8-1.6 5.00 No 1.3 12 80 0 100 2.1Nanjing Oil Storage 1 54,000 24-49 0.7-1.4 0.70 0.4 1.0 18 84 1.1 170 6.7Nanjing Oil Storage 2 27,470 31-57 0.9-1.6 0.70 0.4 0.5 18 80 1.1 0.5 32 8.3Guangzhou Sewage1 16,445 54-75 1.3-2.0 1.2 15 80 0 90 7.9Guangzhou Sewage2 2,900 54-75 1.3-2.0 1.2 15 80 0 1 125 7.5Dalian Bay Alkali 2,025 100-166 2.8-3.6 0.4 1.2 8 80 1.1 56 9.0Zhuhai Power Station 110,000 40-63 1.2-1.7 0.60 0.4 1.0 20 80 0.9 2 149 9.3Nanjiang Coal Terminal 24,550 41-60 0.9-1.6 0.70 0.7 20 80 81 7.2Tianjin Oil Storage 50,000 28-58 2.00 0.3 1.0 20 80 0.4 2 120 4.6Shenzen Wall 957 1.2 12 80 0 1 120 7.1

Area wo Cc/1+eo Ave. cv Sand Drain Treated depth Vacuum Pumping Settlement

(m2) (%) (ave.) x10-3cm2/s blanket(m) Spacing(m) Ave, (m) (kPa) duration (days) Ratio (%)Ariake 400 75 0.27 1.50 0.5 0.80 27 60 35 5.0TohokuRef(2) 194 0.45-0.5 0.81 0.70 10 65 53 8.6TohokuRef(3) 194 0.45-0.5 0.81 0.70 10 65 52 9.2Onogawa 7274 120 0.40 1.20 0.5 0.8-1.0 11.6 60 30 6.3Express Way 337(1) 1036 400-900 0.38 0.58 0.70 20 80 115 13.0Express Way 337(2C) 1466 200-400 0.38 0.58 0.70 20 60 121 21.0Express Way 337(2H) 2616 200-400 0.38 0.58 0.90 20 60 104 11.5Ibaraki 2300 90-750 0.50 2.10 0.80 6.5 80 47 40.0Kochi Exp(P45) 700/70 0.20 3.00 1.00 10 70 120 6.6Kochi Exp(P65) 700/70 0.35 1.50 1.00 10 70 127 15.6Kochi(?) 1800 200/70 0.28 1.50 1.00 10 70 257 27.8Niiga Expw 1671 165/70 0.24 1.50 0.5 1.00 8.3 70 96 16.3Akita 200 0.45 1.15 0.70 9.5 69 145 24.8Hokkaido 4000 260-450 0.7-0.8 20.5 70 112 12.5Nagano 1000 470-530 1.27 0.75 8 40 50 6.3Tohoku WaterTreat 10000 400-600 0.45-0.5 0.70 10 70 48 9.5Bridge Abutment A 2514 708-175 0.53 0.5 0.80 6.3 70Bridge Abutment B 1671 228-103 0.24 0.5 1.00 8.3 70 968.0

13.09.2

5.07.713.68.0

6.013.16.07.2

b) China Vacuum Consolidation with Prefabricated Vertical Drains (PVD)

c) Japan Vacuum Consolidation with Prefabricated Vertical Drains (PVD) Embankment

+surcharge (m)

a) Menard Vacuum Consolidation with vertical transmission pipes (cylindrical VTP)

Project

Soil Properties Vacuum consolidation design

ハザマ研究年報（2006.12） 11

runways and seawalls rather than limited for the port structures. By 1996, the total area treated by using the vacuum method was estimated of more than 2 millions m2 in China. China also extensively employed the method to numerous ground improvement projects in foreign countries.

The East Pier of Xingang Port of Tianjin project conducted by the First Navigational Engineering Bureau of China is the largest vacuum consolidation treatment on reclaimed land of total area of 480,000 m2 situated over 20m-thick soft clay (Shang et all, 1998). The treatment area was divided into 70 blocks each covering 5,000-30,000m2 for individual execution of vacuum consolidation. The treatment covers 4m thick very soft hydraulic fill overlaying soft highly compressible peat and organic clays extended to the depth of 20m. The used vacuum consolidation system with PVDs providing vacuum pressure efficiency of 70 % near ground surface (reduced to 50% at larger depth) was successful to bring a total settlement of 1.6-2.3m (induced both by pretreatment and vacuuming) after 135-175 days of pumping. The soil strength increased up to 1700-2300 % at surface and 30-40% at bottom of the treated zone.

4.2 Practices in Europe

In Sweden as well as other Scandinavian countries due to large distribution of very soft sensitive clays, since introduction of vacuum consolidation its applications are mainly in stabilizing the clayey soil foundations of road embankments or buildings. Vacuum consolidation is also popularly employed in other European countries especially France, Germany, Netherlands, Belgium… Utilizing the popular Wick drains, Cofra & Geotechnics Holland BV (Netherlands) conducted various vacuum consolidation projects in Netherlands, including stabilizing very large embankments located near existing buildings to prevent horizontal deformations in the subsoil during embankment construction, expediting large settlement of landfills, dewatering of chemical waste for chemical plants, and expanding its practice to many other developing countries in Asia.

Menard Sol-Traitment – a French company with its oversea branches in America, Europe and Asia developed its own system named Menard Vacuum Consolidation (MVC) in 1989 and since then has gained lots of experiences in design and construction practice through

numbers of projects in France and in foreign countries (Cognon et al., 1996). The MVC projects involve road and highway embankments (France, Malaysia), bridge structure (Canada), oil tank farms (France), airport terminal (France), port storage area and warehouse (Germany), wharf (Malaysia), power plant (Thailand) and sewage treatment plants (South Korea) (Masse F. et al., 2001). The Highway A837 (France) is typical domestic project of Menard Sol-Traitment, and its recently completed Kimhae Sewage Treatment Plant project in South Korea has become the worldwide record of successful vacuum consolidation project with the deepest ever treatment depth and highest applied load by that time.

In Kimhae Sewage Treatment Plant project (South Korea, 1995), vacuum consolidation in combination with surcharge and following dynamic compaction have been used successfully to allow construction of the sewage treatment plant on an area of 83,580m2 located on 25-43m of highly compressible marine clay deposit. The treatment should guarantee a long-term residual settlement of less than 10cm over 10 years under the design load of 33-155 kN/m2.

The total area had been divided into four sections for treatment separately by employing the MVC system with Menard cylindrical pipes installed at 0.9-1.4m spacing down to the depth of 25-43m. Total of 12 vacuum pumping stations were used (about 7,500m2/pump). An impervious slurry wall as deep as 9m was constructed to prevent leakage from near surface sandy silt layer. Over 9 months of maintaining 70 kPa vacuum pressure in combination with multi-stage surcharge embankment up to 6.5-15.5m (including 1.5m-thick primary fill applied on the top of 1m-thick sand blanket and beneath airtight membrane), consolidation under the design load has been achieved successfully with average settlement of 4.5m over the area (maximum 6.5m). After completion of vacuum treatment and surcharge removal to a final grade level, dynamic compaction and dynamic inclusion were following to improve the bearing capacity of the fill layer. 20 months after soil improvement, the plant entered its full operation with no residual settlement.

4.3 Practices in Japan

In Japan, although investigation on vacuum consolidation technique had been started quite early since

ハザマ研究年報（2006.12） 12

Figure 3 Air water separation vacuum pump system (after Sandanbata et al., 2004)

1960s, however it could have been put in practical applications only since the last two decades when Maruyama-Kougyo Co. Ltd and Hazama Corporation (N&H group) introduced a renewal vacuum consolidation technique called the N&H method. Since then, utilizing newly developed high-tech materials for drains and airtight sheets, the technique has been successfully applied in many domestic projects for ground improvement. Recently, the N&H vacuum consolidation technique has been advanced with utilization of air-water separation vacuum pump system, which ensures a stably high under-sheet vacuum pressure up to 90 kPa during the entire treatment period, allowing shortening the pumping period and thus lowering the cost.

Typical feature of the air-water separation system schemed in Figure 3 is the use of double air-water separation tanks under the airtight membrane. The system works in a manner that the mix of air and water collected by vertical drains, horizontal drains and perforated collector pipes are first led to the supplementary air-water separating tank for being separated, from there the separated water is pumped back by a build-in water pump to the main separating tank through suction hose. Similar separation of the collected air-water mix is also took place in the main tank, where another built-in water pump is responsible for pumping both the just separated water and the water pumped back from the supplementary tank to the vacuum driving system to discharge them outside. The air-water separating tanks are laid beneath the airtight sheet to increase the water discharging efficiency.

Instead of employing the perforated pipe for both collecting and discharging water to vacuum driving system as in previous system, utilization of the build-in water discharge pump in the advanced N&H system helps to overcome the difficulty of maintenance of high vacuum pressure over the entire treatment process when settlement

became large and water head dropped down. N&H group had employed this high efficiency vacuum system in several domestic projects.

A typical project was Sanriku Motorway (Tohoku, Japan 2003) - high vacuum consolidation supporting rapid embankment.

In this project, a 12.5m high experimental embankment covering an area of 3750m2 was constructed on very soft ground consisting of unevenly thick clay and peat layers at the site near Monou Interchange of Sanriku Motorway (Tohoku, Japan). Among various techniques considered, N&H method is the most applicable as it is cost-effective, environmental-friendly and allowing shorten construction time.

The area was divided into 2 blocks for individual treatment. Vacuum consolidation was designed with 70cm thick gravel mat, a system of PVD (type KD-100, 7mm thickness) installed in square grids at 0.8m spacing to the depth of 6-11m depending on the thickness of the soft soil layers. After layout of the air-water separation tanks and perforated collector pipe as well as horizontal board drains (type KD-300) connected to the top of each vertical drain, the area was covered by a special protecting sheet and then an airtight membrane. The membrane edges were sealed off in 2.2m-deep peripheral trench (1.5m into peat layer) and backfilled by the excavated peat.

After layout of vacuum consolidation system and instrumentation, vacuum loading was applied continuously in three stages to 1) improve the initial soil properties, 2) support rapid embankment, and 3) accelerate consolidation after completion of embankment. By utilization of the air water separation system, vacuum pressure beneath the airtight sheet could be attained at record high level of 80-95 kPa and stable during the entire process. Under such high vacuum pressure (about 20-30 kPa higher than designed) the vacuum loading period for

Vert. & Hor.drains

Sub-separator tank Sub-separator

tank

Main-separatortank

Discharge pump

Air ←

Perforated Collector pipe

Vacuum

generator

←Water

←

Water

Watertransmission pipe

Airtight sheet

Air + water←

ハザマ研究年報（2006.12） 13

initial soil improvement could be as short as 30 days, allowing earlier start of embankment, as well as shortening the period of vacuum maintenance after embankment. Consequently the total construction time was reduced and construction cost was lowered. Continuous rapid embankment was executed at an average rate of 13cm/day. The 12.5m height embankment was successfully completed under support of vacuum application for 109 days and maintenance thereafter for 33 more days until complete dissipation of excess pore water pressure. The resulted total settlement by combined vacuum loading and embankment loading was 302 cm in the central area of the embankment.

4.4 Practices in the United States

The US is leading in the field of application of vacuum consolidation for acceleration dewatering and consolidation of hydraulic dredged materials for gaining storage capacity of dumping facilities or reducing the volume of the materials before transportation or treatment.

Sub-aqueous Confined Disposal Facility (CDF) at Newark Bay, New Jersey (U.S. 1997)

The Port of New York and New Jersey constructed a sub-aqueous CDF to contain 1.5 million cubic yards of Category II and Category III contaminated dredged sediments. A study was conducted beforehand to determine the feasibility of using vacuum consolidation to accelerate dewatering of dredged material to gain the capacity of the CDF (Sandiford et al., 1996). In this study, a pit of 2,600,000m3 in volume was excavated under water at the depth of 20m. An engineering sand layer and horizontal slotted pipes were placed at bottom of the pit before dumping of dredged materials (very soft to fluid mud). Vertical drains (spacing 1.5m) capped at the top were installed down to the bottom sand, then vacuum pressure was applied from bottom by a submerged vacuum pump. The system is sealed by the top soil layer to prevent short circuiting to the bay above. With vacuum application over 2 years, settlement of 8.2m was recorded inducing an increase of 855,000m3 in capacity of the CDF. Compared to the other method, vacuum consolidation provides the most capacity and at least cost (US$ 9.5/m3).

5. Conclusions

Numbers of published documents and research papers

on vacuum consolidation practice from different countries in the world with typical vacuum consolidation system configurations and design criteria are reviewed.

Actual design parameters from various actual projects of soft ground improvement conducted in Japan, China, France and other countries were sited or interpreted from published documents or papers, then tabulated and plotted in graphs for comparison.

Furthermore, typical vacuum consolidation systems developed for accelerated dewatering fluid-like materials including hydraulic dredged fill are also reviewed with typical example on system with vertical drains or systems with lateral drains.

Finally, the paper summarizes application practices in leading countries with typical case histories for illustration of effectiveness of the vacuum consolidation method. And the new advanced system (air-water separation N&H Method) is introduced.

References

1) Cognon J.M., Juran I. and Thevanayagam S. (1996),

Vacuum consolidation technology-principles and field

experience. Proc. of Int. Workshop on Technology

Transfer for Vacuum Induced Consolidation: Engineering

and Practice, Los Angeles 1996, pp.201-212

2) Harvey J.A.F. (1997), Vacuum drainage to accelerate

submarine consolidation at Chek Lap Kok, Hongkong.

Ground Engineering, 1997, No.30, pp.34-36

3) Holm G. (1996), Consolidation and stabilization of soft

clays. Proc. of Int. Workshop on Technology Transfer for

Vacuum Induced Consolidation: Engineering and Practice,

Los Angeles 1996, pp.67-75

4) Kjellman W. (1952), Consolidation of clay soil by means

of atmospheric pressure. Proc. of Conference on Soil

Stabilization, Cambridge, Mass., 1952. pp.258-263

5) Jacob A. et al., (1996), Vacuum assisted consolidation of a

hydraulic landfill. Proc. of Int. Workshop on Technology

Transfer for Vacuum Induced Consolidation: Engineering

and Practice, Los Angeles 1996, pp.35-50;

6) Liu Y. (1996), Application and experience of Vacuum

preloading method, TPEI Report

7) MEBRA DRAINS Vacuum Drainage System

(http://www.cofra.com/vacuum.html)

ハザマ研究年報（2006.12） 14

8) Masse F., Spaulding C.A, Wong I.C and Varaskin S.

(2001), Vacuum consolidation- A review of 12 years of

successful development, Geo-Odessey-ASCE/ Virginia

Tech-Blacksburg, VA USA, 2001

9) Sasaki T. (2002), Environment-friendly treatment of very

soft clayey sediments in lakes and marshes, Electric Power

Civil Engineering, 2002.3, pp.115-118 (in Japanese)

10) Sandanbata I., Kimura M., Matsumoto K., Sato Y.,

Nakakuma K., Ichikawa H., (2004), A study for the

stability of the embankment with vacuum preloading

method and the surrounding ground displacement, Proc. of

39th Japan National Conf. on Geotechnical Engineering,

pp.951-952 (in Japanese)

11) Sandiford R.E., Ludewig N, and Dunlop P., (1996),

Evaluation of enhanced consolidation for dredged material

disposed at Newark Bay Confined Disposal Facility Proc.

of Int. Workshop on Technology Transfer for Vacuum

Induced Consolidation: Engineering and Practice, Los

Angeles 1996, pp.27-33

12) Shang J.Q., Tang M., and Miao Z. (1998), Vacuum

preloading consolidation of reclaimed land: a case study.

Canadian Geotechnical Journal., Vo.35, pp.740-749

13) Shinsha H., Watari, Y. and Kurumada, Y. (1996),

Improvement of Very Soft Ground by Vacuum

Consolidation Using Horizontal Drains, Proc. of Int.

Workshop on Technology Transfer for Vacuum Induced

Consolidation: Engineering and Practice, Los Angeles

1996, pp.113-118

14) Shinsha H., Yoneya H., Shiina T., Kiyama M. (2002),

Laboratory experiments on the effect of improvement of

undersea ground by vacuum consolidation method. Proc.

of 37th Japan National Conf. on Geotechnical Engineering,

pp.1065-1066(in Japanese)

15) Tang M. & Shang Q. (2000), Vacuum preloading

consolidation of Yaoqiang Airport runway. Geotechnique

50, No.6, pp.613-623

16) Technical Association on Vacuum Consolidation,

Technical manual for High Vacuum N&H compulsory

dewatering consolidation method – Advanced vacuum

consolidation method -, 2nd edition, 2004.12. (in

Japanese)

17) Thevanayagam S., et al., (1996), Prospects of

vacuum-assisted consolidation for ground improvement of

coastal and off-shore fills. Proc. of Int. Workshop on

Technology Transfer for Vacuum Induced Consolidation:

Engineering and Practice, Los Angeles 1996, pp.35-50

18) TPEI Report (1995), Vacuum preloading method to

improve soft soils and case studies (Tianjin Port

Engineering Institute)

19) Van Mieghem J. et al. (1999), Building on Soft Soils,

Terra-et-Aqua, Int. journal on Public Works, Ports &

Waterways Development

(http://www.terra-et-aqua.com)

真空圧密工法 －世界各国の施工事例と日本における最新技術

ダム ティー キム ローン，三反畑勇，木村 誠

真空圧密工法は，軟弱な粘性土地盤に真空圧（負圧）を作用させて間隙水圧を低下させることによって，全

応力を変化させずに有効応力を増加させる地盤改良技術である。近年の設計・施工技術の向上によって，効率

的で経済的な軟弱地盤対策工法として認められ，施工実績が増加している。また，泥土の脱水・圧密促進とい

った分野にも応用範囲を拡げている。真空圧密工法はこれまでに世界各国で多くの施工事例があり，施工実績

の蓄積が技術の進歩に大いに貢献してきた。本文では，それらの実績から有用な情報を得るため，真空圧密工

法の多様なシステムをその特徴及び設計・施工面から整理，比較するとともに，日本国内における最新の開発

技術について紹介する。